Einsteinium is a synthetic chemical element; it has symbol Es and atomic number 99 and is a member of the actinide series and the seventh transuranium element.
Einsteinium was discovered as a component of the debris of the first hydrogen bomb explosion in 1952. Its most common isotope, einsteinium-253 (Es; half-life 20.47 days), is produced artificially from decay of californium-253 in a few dedicated high-power nuclear reactors with a total yield on the order of one milligram per year. The reactor synthesis is followed by a complex process of separating einsteinium-253 from other actinides and products of their decay. Other isotopes are synthesized in various laboratories, but in much smaller amounts, by bombarding heavy actinide elements with light ions. Due to the small amounts of einsteinium produced and the short half-life of its most common isotope, there are no practical applications for it except basic scientific research. In particular, einsteinium was used to synthesize, for the first time, 17 atoms of the new element mendelevium in 1955.
Einsteinium is a soft, silvery, paramagnetic metal. Its chemistry is typical of the late actinides, with a preponderance of the +3 oxidation state; the +2 oxidation state is also accessible, especially in solids. The high radioactivity of Es produces a visible glow and rapidly damages its crystalline metal lattice, with released heat of about 1,000 watts per gram. Studying its properties is difficult due to Es's decay to berkelium-249 and then californium-249 at a rate of about 3% per day. The longest-lived isotope of einsteinium, Es (half-life 471.7 days) would be more suitable for investigation of physical properties, but it has proven far more difficult to produce and is available only in minute quantities, not in bulk.Einsteinium . periodic.lanl.gov Einsteinium is the element with the highest atomic number which has been observed in macroscopic quantities in its pure form as einsteinium-253.
Like all synthetic transuranium elements, isotopes of einsteinium are very radioactive and are considered highly dangerous to health on ingestion.
History
Einsteinium was first identified in December 1952 by Albert Ghiorso and co-workers at University of California, Berkeley in collaboration with the Argonne and Los Alamos National Laboratories, in the fallout from the Ivy Mike nuclear test. The test was done on November 1, 1952, at Enewetak Atoll in the Pacific Ocean and was the first successful test of a thermonuclear weapon. Initial examination of the debris from the explosion had shown the production of a new isotope of plutonium, 244Pu, which could only have formed by the absorption of six neutrons by a uranium-238 nucleus followed by two beta decays. ^{238}_{92}U ->[\ce{+ 6(n,\gamma)}][-2\ \beta^-]{} ^{244}_{94}Pu At the time, the multiple neutron absorption was thought to be an extremely rare process, but the identification of Pu indicated that still more neutrons could have been captured by the uranium, producing new elements heavier than californium.
Ghiorso and co-workers analyzed filter papers which had been flown through the explosion cloud on airplanes (the same sampling technique that had been used to discover Pu).Seaborg, p. 39 Larger amounts of radioactive material were later isolated from coral debris of the atoll, and these were delivered to the U.S. The separation of suspected new elements was carried out in the presence of a citric acid/ammonium buffer solution in a weakly acidic medium (pH ≈ 3.5), using ion exchange at elevated temperatures; fewer than 200 atoms of einsteinium were recovered in the end. Nevertheless, element 99, einsteinium, and in particular Es, could be detected via its characteristic high-energy alpha decay at 6.6 MeV. It was produced by the capture of 15 neutrons by uranium-238 nuclei followed by seven beta decays, and had a half-life of 20.5 days. Such multiple neutron absorption was made possible by the high neutron flux density during the detonation, so that newly generated heavy isotopes had plenty of available neutrons to absorb before they could disintegrate into lighter elements. Neutron capture initially raised the mass number without changing the atomic number of the nuclide, and the concomitant beta-decays resulted in a gradual increase in the atomic number: ^{238}_{92}U ->[\ce{+15n}][6 \beta^-] ^{253}_{98}Cf ->[\beta^-] ^{253}_{99}Es
Some U atoms, however, could absorb two additional neutrons (for a total of 17), resulting in Es, as well as in the Fm isotope of another new element, fermium.Es, Fm and Fm would not be produced because of lack of beta decay in Cf and Es. The discovery of the new elements and the associated new data on multiple neutron capture were initially kept secret on the orders of the U.S. military until 1955 due to Cold War tensions and competition with Soviet Union in nuclear technologies. However, the rapid capture of so many neutrons would provide needed direct experimental confirmation of the r-process multi-neutron absorption needed to explain the cosmic nucleosynthesis (production) of certain heavy elements (heavier than nickel) in supernovas, before beta decay. Such a process is needed to explain the existence of many stable elements in the universe. (pbk.) pp. 267.
Meanwhile, isotopes of element 99 (as well as of new element 100, fermium) were produced in the Berkeley and Argonne laboratories, in a nuclear reaction between nitrogen-14 and uranium-238, and later by intense neutron irradiation of plutonium or californium: ^{252}_{98}Cf ->[\ce{(n,\gamma)}] ^{253}_{98}Cf ->[\beta^-][17.81 \ce{d}] ^{253}_{99}Es ->[\ce{(n,\gamma)}] ^{254}_{99}Es ->[\beta^-] ^{254}_{100}Fm
These results were published in several articles in 1954 with the disclaimer that these were not the first studies that had been carried out on the elements. The Berkeley team also reported some results on the chemical properties of einsteinium and fermium. Radiation Laboratory, University of California, Berkeley, UCRL-2591 The Ivy Mike results were declassified and published in 1955.
In their discovery of elements 99 and 100, the American teams had competed with a group at the Nobel Institute for Physics, Stockholm, Sweden. In late 1953 to early 1954, the Swedish group succeeded in synthesizing light isotopes of element 100, in particular Fm, by bombarding uranium with oxygen nuclei. These results were also published in 1954. Nevertheless, the priority of the Berkeley team was generally recognized, as its publications preceded the Swedish article, and they were based on the previously undisclosed results of the 1952 thermonuclear explosion; thus the Berkeley team was given the privilege to name the new elements. As the effort which had led to the design of Ivy Mike was codenamed Project PANDA, element 99 had been jokingly nicknamed "Pandemonium" but the official names suggested by the Berkeley group derived from two prominent scientists, Einstein and Fermi: "We suggest for the name for the element with the atomic number 99, einsteinium (symbol E) after Albert Einstein and for the name for the element with atomic number 100, fermium (symbol Fm), after Enrico Fermi." Both Einstein and Fermi died between the time the names were originally proposed and when they were announced. The discovery of these new elements was announced by Albert Ghiorso at the first Geneva Atomic Conference held on 8–20 August 1955. The symbol for einsteinium was first given as "E" and later changed to "Es" by IUPAC.Haire, p. 1577
Characteristics
Physical
Einsteinium is a synthetic, silvery, radioactive metal. In the periodic table, it is located to the right of the actinide californium, to the left of the actinide fermium and below the lanthanide holmium with which it shares many similarities in physical and chemical properties. Its density of 8.84 g/cm is lower than that of californium (15.1 g/cm) and is nearly the same as that of holmium (8.79 g/cm), despite einsteinium being much heavier per atom than holmium. Einsteinium's melting point (860 °C) is also relatively low—below californium (900 °C), fermium (1,527 °C) and holmium (1,461 °C).Hammond C. R. "The elements" in Haire, R. G. (1990) "Properties of the Transplutonium Metals (Am-Fm)", in: Metals Handbook, Vol. 2, 10th edition, (ASM International, Materials Park, Ohio), pp. 1198–1201. Einsteinium is a soft metal, with a bulk modulus of only 15 GPa, one of the lowest among non-alkali metals.Haire, p. 1591
Unlike the lighter actinides californium, berkelium, curium and americium, which crystallize in a double hexagonal structure at ambient conditions; einsteinium is believed to have a face-centered cubic (fcc) symmetry with the space group Fmm and the lattice constant . However, there is a report of room-temperature hexagonal einsteinium metal with and , which converted to the fcc phase upon heating to 300 °C.
The self-damage induced by the radioactivity of einsteinium is so strong that it rapidly destroys the crystal lattice, and the energy release during this process, 1000 watts per gram of 253Es, induces a visible glow.Haire, p. 1579 These processes may contribute to the relatively low density and melting point of einsteinium. draft manuscript Further, due to the small size of available samples, the melting point of einsteinium was often deduced by observing the sample being heated inside an electron microscope.Seaborg, p. 61 Thus, surface effects in small samples could reduce the melting point.
The metal is trivalent and has a noticeably high volatility. In order to reduce the self-radiation damage, most measurements of solid einsteinium and its compounds are performed right after thermal annealing.Seaborg, p. 52 Also, some compounds are studied under the atmosphere of the reductant gas, for example HO+HCl for EsOCl so that the sample is partly regrown during its decomposition.
Apart from the self-destruction of solid einsteinium and its compounds, other intrinsic difficulties in studying this element include scarcity—the most common Es isotope is available only once or twice a year in sub-milligram amounts—and self-contamination due to rapid conversion of einsteinium to berkelium and then to californium at a rate of about 3.3% per day:Seaborg, p. 55 ^{253}_{99}Es ->[\alpha][20 \ce{d}] ^{249}_{97}Bk ->[\beta^-][314 \ce{d}] ^{249}_{98}Cf
Thus, most einsteinium samples are contaminated, and their intrinsic properties are often deduced by extrapolating back experimental data accumulated over time. Other experimental techniques to circumvent the contamination problem include selective optical excitation of einsteinium ions by a tunable laser, such as in studying its luminescence properties.Seaborg, p. 76
Magnetic properties have been studied for einsteinium metal, its oxide and fluoride. All three materials showed Curie–Weiss paramagnetic behavior from liquid helium to room temperature. The effective magnetic moments were deduced as for EsO and for the EsF, which are the highest values among actinides, and the corresponding Curie temperatures are 53 and 37 K.
Chemical Like all actinides, einsteinium is rather reactive. Its trivalent oxidation state is most stable in solids and aqueous solution where it induces a pale pink color.Holleman, p. 1956 The existence of divalent einsteinium is firmly established, especially in the solid phase; such +2 state is not observed in many other actinides, including protactinium, uranium, neptunium, plutonium, curium and berkelium. Einsteinium(II) compounds can be obtained, for example, by reducing einsteinium(III) with samarium(II) chloride.Seaborg, p. 53
Isotopes